The present disclosure relates to nano-scale ferromagnets (nanomagnets), and more particularly to use of ferromagnet logic systems in sequential logic applications.
With CMOS scaling approaching fundamental limits, emerging logic devices based on novel state variables are considered promising candidates for beyond-CMOS computation solutions. So-called spintronic devices can be used to implement binary logic functionality. Magnetic anisotropy is a directional dependence of the magnetic properties of the magnet material. The magnetization direction of nanomagnets is the collective polarization direction of all the spins inside of the magnets. Rectangular or elliptical shaped nanomagnets have lower energy when the magnetization direction is along the long side of the magnet configuration than when the magnetization direction is along the short side. The magnetic material will align its magnetic moment to the lower energy long direction, commonly referred to as the “easy” axis. The higher energy direction, i.e., the short side, is referred to as the “hard” axis.
Logic information can be represented by the magnetization direction of ferromagnets 10 and 12, as shown in FIG. 1(a). In the illustrated rectangular configuration, the easy axes are in the vertical direction. The magnetization direction can be upwards, as shown by the arrow in magnet 10 or downwards, as shown by the arrow in magnet 12. The upward and downward directions are binary complements. The upward direction may be arbitrarily selected to represent a logic “0”, the downward direction representing a logic “1”. Energy applied to a magnet, such as by imposition of a magnetic field in the hard axis direction, can temporarily change the magnetization direction.
An energy barrier differentiates the two nonvolatile logic states, as illustrated in FIG. 1(b), wherein energy is plotted with respect to the angle of magnetization. The curve indicates an energy maximum, or barrier, at the hard axis angle. The energy difference is known as the shape anisotropy energy of the magnet, which is proportional to the anisotropy constant of the magnet material multiplied by the magnet volume. The logic state of the magnet can be “switched” by application of barrier energy in the hard axis direction to temporarily drive the magnetization direction toward the hard axis and applying a field that orients the magnetization in either (up or down) easy axis direction.
Ferromagnet logic operation is reliant on the magnetic field coupling between neighboring nanomagnets. Magnets closely placed together interact with each other via the short-ranged magnetostatic dipole fields. The fringing magnetic field of one nanomagnet can affect the orientation of its neighboring nanomagnets. FIG. 2 shows a chain of nanomagnets antiferromagnetically coupled to each other. The dashed lines indicate the fringing magnetic fields of the magnets with the arrows pointing to the directions of the magnetic fields. Logic information encoded in the magnetization direction of each nanomagnet can propagate through the chain of nanomagnets.
FIGS. 3(a-c) illustrate nanomagnetic switching operation facilitated by application of external field B. As shown in FIG. 3(a), a fixed input field is applied to input magnet 10 in a direction corresponding to the required logic state. A field B is applied along the hard axis direction. Field B is of sufficient magnitude to deflect the magnetization directions of magnets 12-14 in the hard axis direction of the applied field. FIG. 3(c) is a plot representing the energy landscapes of the magnets with and without the applied external field. The energy barrier between the two logic states is lowered by field B, thereby making logic transition easier. As the magnetization of the nanomagnets is unstable in this direction, when the external field B is removed the easy axis magnetization directions of the nanomagnets will be decided by the fringe field of the input nanomagnet 10.
FIG. 3(b) illustrates the magnetization directions of magnets 10-14 after removal of the clocking and fixed input fields. Removal of the clocking field effects deflection of the magnetic polarization directions of the magnets from the hard axis horizontal direction to the easy axis vertical direction. As, in this example, the fixed input applied to magnet 10 causes an upward deflection of magnetization direction, or a logic 0, the coupling between the successive neighboring magnets 12-14, effects alternate magnetization directions along the chain of magnets. Since there is an even number of magnets, the magnetization direction of magnet 14 is downward, corresponding to a logic 1. If magnet 14 is sensed as an output, the illustrated chain of magnets functions as a logic inverter.
Although each nanomagnet is a nonvolatile element that can store information, the magnets can each be disturbed by noise/error and external fields. The length of a magnet chain along which magnetization information can propagate without error is limited. As a logic signal can propagate in the forward or backward direction in a chain of nanomagnets, direction control is a significant challenge. In order to implement sequential logic, a logic signal needs to be stored in each logic operation stage and retrieved in the next operation stage. The need thus exists for improved signal control and synchronization control in ferromagnet logic systems. To advance this need, special elements should be developed for insertion in ferromagnet logic systems to control signal flow and store logic information.